The removal of volatile organic compounds (VOCs) from air or nitrogen is an important step in many industrial manufacturing processes in order to meet emission regulations, recover and recycle valuable reactants or solvents, and reuse gases such as nitrogen in the manufacturing process. This procedure is widely used for example in the petrochemical and pharmaceutical industries to treat dry streams containing one or more VOCs.
Volatile compounds can be removed from gas streams by several different methods. Among the oldest of methods is to compress and cool the gas stream, and expand the compressed stream for further cooling by autorefrigeration as disclosed by U.S. Pat. Nos. 575,714 and 1,040,886. Condensable components are removed therefrom at appropriate temperatures to avoid freezing.
Removal of volatile compounds by adsorption on solid adsorbents or by absorption in suitable liquids, followed by regeneration or distillation to recover the volatile components, are well-known methods as summarized for example in the Encyclopedia of Chemical Technology, Third Edition, Volume 21, John Wiley & Sons, 1983, pp. 355-376. Cooling by ambient cooling water or mechanical refrigeration can be used to supplement these methods.
Liquid nitrogen is used as a refrigeration source for volatile component recovery in a number of processes. Indirect cooling, in which the gas stream containing the volatile compounds is cooled by indirect heat exchange between the gas stream and vaporizing liquid nitrogen, is disclosed in U.S. Pat. Nos. 4,150,494, 4,237,700, and 5,214,924 and French Patent Publication No. 2,349,113. Another type of indirect cooling is disclosed in U.S. Pat. No. 4,545,134 in which vaporizing liquid nitrogen indirectly cools a recirculating stream of an intermediate heat transfer fluid such as toluene, which in turn cools a process stream containing residual volatile components. U.S. Pat. Nos. 4,444,016 and 4,545,134 teach the use of direct contact refrigeration using liquid nitrogen which is contacted with condensed vapor, which is used in turn to contact and cool the gas containing the vapor components. Mechanical refrigeration is used to precool the gas. All of the methods described above which use liquid nitrogen as the refrigerant are characterized by operation at pressures slightly above atmospheric. Similarly, all of the methods described above in which the VOC-laden gas is indirectly cooled to effect condensation operate at pressures slightly above atmospheric, which pressures are generated by the use of fans or blowers.
The presence of water with the volatile organic compounds in the gas stream can cause undesirable freezing, and the presence of water generally complicates the operation of recovery systems such as those described above. In addition, when extremely low concentrations of volatile components or high levels of recovery are required in the final purified gas, multiple stages must be used at successively lower temperatures, and these temperatures must be controlled carefully if water is present. It is desirable to minimize the number of such stages to reduce capital cost of the recovery system.
The method of the present invention, described in the following specification and defined in the claims which follow, addresses these problems in the recovery of volatile components from low-boiling gases, particularly when water is present and extremely low concentrations of volatile components are required in the final purified gas.